CMYK describes how much cyan, magenta, yellow, and black ink to lay down to produce a color on paper — each channel 0 to 100%. It's the print industry's color model because printers add ink (subtractive color) instead of emitting light (additive RGB). The same color almost always renders slightly differently on-screen vs in print because the gamut differs. Required by every commercial print shop and built into Photoshop's print prep workflow.
How to open
Adobe Photoshop, Illustrator, InDesign all natively work in CMYK mode (Image → Mode → CMYK Color). Affinity Photo, GIMP via plugins. Print-shop submission portals accept CMYK PDFs directly.